Heteroepitaxal fabrication and structural characterizations of ultrafine GaN/ZnO coaxialnanorod heterostructuresSung Jin An, Won Il Park, Gyu-Chul Yi, Yong-Jin Kim, Hee-Bok Kang, and Miyoung Kim Citation: Applied Physics Letters 84, 3612 (2004); doi: 10.1063/1.1738180 View online: http://dx.doi.org/10.1063/1.1738180 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/84/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Structural and resistance switching properties of epitaxial Pt/ZnO/TiN/Si(001) heterostructures J. Appl. Phys. 115, 234501 (2014); 10.1063/1.4883959 Microstructural compositional, and optical characterization of GaN grown by metal organic vapor phase epitaxyon ZnO epilayers J. Vac. Sci. Technol. B 27, 1655 (2009); 10.1116/1.3137967 Strain relaxation in AlGaN multilayer structures by inclined dislocations J. Appl. Phys. 105, 083507 (2009); 10.1063/1.3087515 Nanostructural characterization and two-dimensional electron-gas properties in high-mobility Al Ga N ∕ Al N ∕ GaN heterostructures grown on epitaxial AlN/sapphire templates J. Appl. Phys. 98, 063713 (2005); 10.1063/1.2060946 Structural characterization of strained AlGaN layers in different Al content Al Ga N ∕ Ga N heterostructures and itseffect on two-dimensional electron transport propertiesa) J. Vac. Sci. Technol. B 23, 1527 (2005); 10.1116/1.1993619

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Heteroepitaxal fabrication and structural characterizations of ultrafineGaNÕZnO coaxial nanorod heterostructures

Sung Jin An, Won Il Park, and Gyu-Chul Yia)

Department of Materials Science and Engineering, Pohang University of Science and Technology(POSTECH), Pohang 790-784, Korea

Yong-Jin Kim and Hee-Bok KangLG Siltron Advanced Materials Research Center, 283, Imsoo-dong, Gumi, Kyoungbuk 730-724, Korea

Miyoung KimSamsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, Korea

~Received 16 February 2004; accepted 10 March 2004; published online 20 April 2004!

We report on heteroepitaxial fabrication and structural characterizations of ultrafine GaN/ZnOcoaxial nanorod heterostructures. The coaxial nanorod heterostructures were fabricated by epitaxialgrowth of a GaN layer on ultrafine ZnO nanorods. Epitaxial growth and precise control of GaNoverlayer thickness were obtained by low pressure metalorganic vapor-phase epitaxy. ZnO nanorodsgrown on Si and sapphire substrates using catalyst-free metalorganic chemical vapor depositionexhibited diameters as small as 7 nm. Furthermore, structural properties of the coaxial nanorodheterostructures were investigated using both synchrotron-radiation x-ray diffraction and highresolution transmission electron microscopy. ©2004 American Institute of Physics.@DOI: 10.1063/1.1738180#

Composition-modulated semiconductor nanorod hetero-structures greatly increase the versatility and power ofnanometer-scale electronics and optoelectronic buildingblocks.1–3 Recent fabrication of semiconductor coaxial nano-rod heterostructures showing composition modulation alongthe nanorod radial direction is of particular high interest.4–7

Although coaxial structures exhibiting a thick diameter of20–100 nm have previously been applied to nano-scale highelectron mobility transistors~HEMTs!,5 the device perfor-mance may be enhanced by using ultrafine coaxial hetero-structures due to one-dimensional~1D! quantum confine-ment effects in core nanorods. Additionally, a well-defined,clean interface in the heterostructures is very important forexcellent device performance and reliability since defects atthe interface deteriorate device characteristics. For 1D co-axial heterostructures, in particular, the effect of interfacialdefects on material properties becomes more significant dueto their high interface to volume ratio. Nevertheless, interfa-cial defects in 1D nanorod heterostructures have rarely beeninvestigated to date. Here we report on the synthesis of ul-trafine GaN/ZnO coaxial nanorod heterostructures by het-eroepitaxy and their structural defect characterizations.

GaN/ZnO coaxial nanorod heterostructures were fabri-cated on Al2O3 and Si substrates using catalyst-free metal-organic vapor phase epitaxy~MOVPE!. ZnO nanorods wereprepared as a core nanomaterial using diethylzinc and oxy-gen as the reactants.4,8 After the ZnO nanorod preparation,GaN layers were epitaxially grown on the ZnO nanorodsusing trimethyl-Ga ~TMGa! and ammonia in anotherMOVPE chamber. Typical flow rates of TMGa and ammoniawere 3.0 and 500–1000 sccm, respectively. Growth tempera-

ture was in the range of 400–700 °C. The mean diameters ofGaN/ZnO coaxial nanorod heterostructures were about 14,20, and 32 nm for 1, 5, and 10 min growth, respectively.Since the mean diameter of ultrafine ZnO core nanorods is7–8 nm, the GaN layer thickness is 2–3 and 5–7 nm for 1and 5 min growth, respectively, as also confirmed using highresolution transmission electron microscopy~HRTEM!.4,9

Synchrotron radiation x-ray diffraction~SR-XRD! wasemployed for the structural characterization of coaxial nano-rod heterostructures. The SR-XRD measurements of GaN/ZnO nanorod heterostructures were performed at the 1.54019Å wavelength using a four-circle diffractometer of the 3C2SR-XRD beam line at the Pohang Accelerator Laboratory~PAL!.9

Figure 1 shows the general morphology of GaN/ZnO

a!Author to whom correspondence should be addressed; electronic mail:gcyi@postech.ac.kr

FIG. 1. FESEM image and low magnification TEM image of GaN/ZnOcoaxial nanorod heterostructures~inset!.

APPLIED PHYSICS LETTERS VOLUME 84, NUMBER 18 3 MAY 2004

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nanorod heterostructures investigated using field-emissionscanning electron microscopy~FESEM!. Although the verti-cal alignment of coaxial nanorod heterostructures was not asgood as that of thick ZnO nanorods, the nanorod density was1.631010/cm2. Further structural characterization of GaN/ZnO coaxial nanorod heterostructures was investigated usingTEM. As shown in the inset, the low magnification TEMimage shows that the GaN layer extends all along the ZnOnanorod with different brightness at the core and shell layers,indicating that the GaN layers were uniformly coated onZnO core nanorods. The coating was robust and GaN layerswere not easily peeled off from the ZnO core nanorods. Asindicated by the arrow in the inset, however, some TEMimages show GaN/ZnO coaxial nanorod heterostructure andGaN nanotubes formed during fabrication of the nanorodheterostructures or removal of GaN/ZnO coaxial nanorodheterostructures from the substrate.7

Figure 2 shows SR-XRD results of GaN/ZnO ultrafinenanorod heterostructures with various GaN thicknesses of 0,2, 5, and 12 nm. The diameters of the core ZnO nanorodswere in the range of 6–10 nm. As shown in Fig. 2, homoge-neous core ZnO nanorods showed a dominant diffractionpeak centered at 34.42° corresponding to the ZnO~0002!plane. However, SR-XRD data of the GaN/ZnO coaxial na-norod heterostructures exhibited a slight increase in peak po-sition from 34.42° to 34.69° depending on the GaN layerthickness coated on ZnO nanorods. For GaN/ZnO coaxialnanorod heterostructures with GaN layer thicknesses of 2–3and 5–7 nm, SR-XRD peaks were observed at 34.46° and34.5°, respectively. Moreover, the full width at half maxi-mum ~FWHM! value in XRD rocking curve data increasedwith the GaN layer thickness.

The peak position and FWHM depending on the GaNlayer thickness presumably results from lattice distortionalong thec axis of GaN due to biaxial in-plane strain. Fornanorod heterostructures with a thicker GaN layer, morestrain is induced, hence increasing both XRD peak shift andFWHM values. Since thea- andc-axis lattice constants are3.186 and 5.178 Å for GaN and 3.249 and 5.205 Å for ZnO,respectively, epitaxial GaN layers and ZnO nanorods in theheterostructures must have tensile and compressive strain,respectively.10 Since the thickness of GaN is relatively thin-

ner than the diameter of ZnO core nanorods, SR-XRD datashow the dominant peak from ZnO with a slight peak shiftdue to the compressive strain. As shown in Fig. 2, the peakshift increases with increasing GaN thickness. For SR-XRDdata of the nanorod heterostructures with a GaN layer thick-ness of 12 nm, however, a shoulder at 34.42°, in addition tothe dominant peak at 34.69°, is also shown. Lattice constantscalculated from XRD peaks are 5.205 Å for ZnO and 5.178Å for GaN, which are in excellent agreement with the bulkvalues.10 This separation in the XRD peak for nanorod het-erostructures with a thicker GaN layer is presumably due tostrain release. With the continuous growth of GaN on theZnO nanorod core, GaN layers readjust to bulk GaN param-eters and induced lattice dislocations~semicoherency! at theinterface. This may allow the relief of some of the accumu-lated strain.

Further structural characterization of GaN/ZnO coaxialnanorod heterostructures was investigated using HRTEM. Asshown in Fig. 3~a!, the HRTEM image of a GaN/ZnO coaxialnanorod heterostructure shows an abrupt and semicoherent

FIG. 2. SR-XRDu–2u scan of bare ZnO nanorods and GaN/ZnO coaxialnanorod heterostructures with various GaN thicknesses of 2, 5, and 12 nm.

FIG. 3. ~a! HRTEM image of a GaN/ZnO coaxial nanorod heterostructureand~b! its filtered and inverse FFT image~inset!. The inset in~a! shows anelectron diffraction pattern of the GaN/ZnO coaxial nanorod heterostructure.The peak splitting indicates the slight difference between GaN and ZnOlattice constants. As indicated in the inverse FFT image, the lattice of theGaN layer does not exactly match that of the ZnO layer, due to the forma-tion of dislocation.

3613Appl. Phys. Lett., Vol. 84, No. 18, 3 May 2004 An et al.

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interface between ZnO and GaN layers, verifying that theGaN layers are epitaxially grown on ZnO nanorods. The in-set shows an electron diffraction pattern of a GaN/ZnO co-axial nanorod heterostructure obtained by fast Fourier trans-form ~FFT!. The pattern can be indexed as a@100# zone axishexagonal wurtzite GaN and ZnO, where two diffractionspots labeled in the inset correspond to planes~0002! and(0110). Two peaks corresponding to the (0110) reflectionsof ZnO and GaN are distinguished due to the slight differ-ence in lattice constants along the radial direction, indicatingradial relaxation of GaN, while the peak splitting of~0002!reflection along the growth direction is not observed, indicat-ing existence of strain from epitaxial growth.

In general, the strain between heteroepitaxial layers in-duces formation of defects such as misfit dislocations andstacking faults to release the strain. The SR-XRD resultsshow the presence of strain in the nanorod heterostructures.

Hence we further investigate defects near the interface be-tween ZnO and GaN layers using HRTEM. Despite the ob-servation of the abrupt interface, close examination of theGaN and ZnO lattice fringes reveals the presence of edgedislocations as indicated by the arrows in Fig. 3~a!. This isshown clearly in the inverse-FFT image in Fig. 3~b!, whichindicates the formation of the edge dislocation with a Bur-gers vector in the nanorod growth direction@001#. This typeof edge dislocation is rarely observed in pure ZnO nanorods,but the occurrence is increased in GaN/ZnO coaxial nanorodheterostructures, probably due to the lattice mismatch be-tween both structures or the inherent nature of GaN nanoma-terial.

Planar stacking faults are also often observed in coaxialstructures. Figures 4~a! and 4~b! show the image and its dif-fractogram. The diffractogram obtained from the small whitesquare shows streaks along the@001#, indicating the exis-tence of stacking faults. The density of these defects, whichhas not been observed in pure ZnO nanorods, is about a fewtens per micrometer. These results indicate that GaN wasepitaxially grown on ZnO nanorods, which presumably re-sults from the little lattice mismatch between GaN and ZnO.

In conclusion, ultrafine GaN/ZnO coaxial nanorod het-erostructures were fabricated by heteroepitaxial growth ofGaN on ZnO nanorods. The fabrication of ultrafine coaxialnanorod heterostructures opens up significant opportunitiesfor the fabrication of 1D quantum structure devices includingHEMTs. Furthermore, the structural and defect characteriza-tion methods used in nanorod heterostructures may readilybe expanded to characterize structural defects in many otherheteroepitaxial nanomaterials.

This work was supported by LG Siltron Co., the Na-tional R&D Project for Nano Science and Technology, theAEBRC ~Grant No. R11-2003-006!, the Brain Korea 21project, and the POSRIP-2003.

1W. I. Park, S. W. Jung, Y. H. Jun, and G.-C. Yi, in Proceedings of the 26thInternational Conference on the Physics of Semiconductors, Edinburgh,UK, 2002, p. 176.

2M. S. Gudiksen, L. J. Lauhon, J. Wang, D. C. Smith, and C. M. Lieber,Nature~London! 415, 617 ~2002!.

3W. I. Park, G.-C. Yi, M. Kim, and S. J. Pennycook, Adv. Mater.~Wein-heim, Ger.! 15, 526 ~2003!.

4Y. Zhang, K. Suenaga, C. Colliex, and S. Iijima, Science281, 973~1998!.5L. J. Lauhon, M. S. Gudiksen, D. Wang, and C. M. Lieber, Nature~Lon-don! 420, 57 ~2002!.

6H.-M. Lin, Y.-L. Chen, J. Yang, Y.-C. Liu, K.-M. Yin, J.-J. Kai, F.-R.Chen, L.-C. Chen, Y.-F. Chen, and C.-C. Chen, Nano Lett.3, 537 ~2003!.

7J. Goldberger, R. He, Y. Zhang, S. K. Lee, H. Yan, H.-J. Choi, and P. Yang,Nature~London! 422, 599 ~2003!.

8W. I. Park, D. H. Kim, S.-W. Jung, and G.-C. Yi, Appl. Phys. Lett.80,4232 ~2002!.

9S. W. Jung, W. I. Park, G.-C. Yi, and M. Kim, Adv. Mater.~Weinheim,Ger.! 15, 1358~2003!.

10F. Hamdani, M. Yeadon, D. J. Smith, H. Tang, W. Kim, A. Salvador, A. E.Botchkarev, J. M. Gibson, A. Y. Polyakov, M. Skowronski, and H.Morkoc, J. Appl. Phys.83, 983 ~1998!.

FIG. 4. ~a! TEM image and~b! diffractogram of coaxial nanorod hetero-structures with stacking faults. The diffractogram was obtained from thewhite square.

3614 Appl. Phys. Lett., Vol. 84, No. 18, 3 May 2004 An et al.

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